Photodetectors are an integral part of optical circuits and components (for example emitters, modulators, repeaters, waveguides or fibers, reflectors, resonators, detectors, IR Focal plane arrays, etc.) and are used for the sensing of electromagnetic radiation. There are several approaches to these devices. Photoconducting materials, typically semiconductors, have electrical properties that vary when exposed to electromagnetic radiation (i.e. light). One type of photoconductivity arises from the generation of mobile carriers (electrons or holes) during absorption of photons. For semiconducting materials, the absorption of a specific wavelength of light, hence photon energy, is directly proportional to the band gap of the material (Eg=hn=hc/l, where Eg is the materials band gap, h is Plank's constant (4.136×10-15 eVs), c is the speed of light in a vacuum (2.998×1010 cm/s) and 1 is the wavelength of the radiation). If the band gap energy is measured in eV (electron Volts) and the wavelength in micrometers, the above equation reduces to Eg=1.24/1. A photodiode (i.e. p-n diode, p-i-n photodiode, avalanche photodiode, etc.) is the most commonly employed type of photoconductor.
Light detection is ideally suited for direct band gap semiconductors such as Ge, GaAs, etc.; however, indirect band gap semiconductors (where an additional phonon energy is required to excite an electron from the valence band to the conduction band), such as silicon, are also used as photodetectors. Probably the most widely known type of photodetector is the solar cell, which uses a simple p-n diode or Schottky barrier to detect impinging photons. Besides silicon, most photodetectors do not integrate with current microelectronics technology, usually detect only a specific wavelength (i.e. 1.1 mm for Si, 0.87 mm for GaAs, 0.414 mm for a-SiC and 1.89 mm for Ge), and require multiple detectors to detect a broad band of wavelengths (hence photon energy).
There are other types of photodetectors that do not rely on the generation of current through the excitation of electrons (or holes). One such type of detector is the bolometer. Bolometers operate by absorbing radiation, which in turn raises the temperature of the material and hence alters the resistance of the material. Bolometers can be constructed from either metallic, metallic-oxides or semiconducting materials such as vanadium oxide, amorphous silicon. Since bolometers detect a broad range of radiation above a few microns, bolometers are typically thermally stabilized to reduce the possibility of detection of blackbody radiation that is emitted from the detector material, which leads to a high background noise. IR microbolometer detectors and arrays don't require cooling to cryogenic temperatures unlike the other detector technologies discussed. Another type of non-photo-generated detector is the pyroelectric detector. Pyroelectric detectors operate by sensing induced surface charges that are related to changes in the internal dipole moment generated from temperature shifts in the material.
It is possible for IR and visible light to be detected from individual single-walled graphene and both single wall (SWNTs) and multiwall carbon nanotubes. (Itkis, Mikhail E., Ferenc Borondics, Aiping Yu, and Robert C. Haddon. “Bolometric infrared photoresponse of suspended single-walled carbon nanotube films.” Science 312, no. 5772 (2006): 413-416, and Du, Xu, Ivan Skachko, Anthony Barker, and Eva Y. Andrei. “Approaching ballistic transport in suspended graphene.” Nature nanotechnology 3, no. 8 (2008): 491-495.) Graphene possesses discrete absorption peaks that correspond to specific photon energies. For useful background material, refer to U.S. Pat. No. 6,400,088. As described, the absorption peaks of the graphene correlate directly to the diameter of the carbon nanotube.
Typical band-gaps for graphene range from 0.6-1.2 eV, where the band gap is proportional to the inverse thickness of the layer. These energies correlate to the graphene ability to detect radiation in the near IR range. Since graphene can also generate heat and phonons by several processes (injection of electrons, impinging with radiation, etc.), (Calizo, I., A. A. Balandin, W. Bao, F. Miao, and C. N. Lau. “Temperature dependence of the Raman spectra of graphene and graphene multilayers.” Nano letters 7, no. 9 (2007): 2645-2649.) CNT fabric is also ideally suited as an IR detector.
Thermal infrared detectors, such as microbolometers, are a part of IR systems used to image heat emitted from natural phenomena. The current state of the art microbolometer utilizes vanadium oxide as the element which changes impedance for incoming IR radiation. Despite the improvements to the use and costs of IR focal plane arrays (IRFPAs), which are most sensitive in the LWIR (8-12 microns) and MWIR (3-5 microns), there seems to be a limit to sensitivity at 20 mk NEDT. (Purewal, Meninder S., Byung Hee Hong, Anirudhh Ravi, Bhupesh Chandra, James Hone, and Philip Kim. “Scaling of resistance and electron mean free path of single-walled carbon nanotubes.” Physical review letters 98, no. 18 (2007): 186808.)
This performance is restricted by 1/f noise and the basic physical properties of the vanadium oxide (VOx) film. In addition, as the needs for increased sensitivity and smaller pixel size below 25 micron, silicon technology has run up against a sensitivity wall due to the scaling of 1/f noise as pixel size are reduced and the absolute noise floor is realized the basic properties of the silicon needs to be optimized for low noise operation.
One solution of the prior art is to use of carbon nanotubes to reduce noise, as described in U.S. Pat. No. 8,110,883, which includes the generations of excitons to produce heat in the IR sensing element to change the TCR response and thereby sensitivity. Carbon nanotubes also have high absorption coefficients of 10−4 to 10−5, which is higher than HgCdTe in the 8-12 micron region.